With the development of radio and space technologies, several satellite-based navigation systems (i.e. satellite positioning system or SPS) have already been built and more will be in use in the near future. SPS receivers, such as, for example, receivers using the Global Positioning System (GPS), also known as NAVSTAR, have become commonplace. Other examples of SPS systems include, but are not limited to, the United States (U.S.) Navy Navigation Satellite System (NNSS), the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (GLONASS), and any future Western European SPS such as the proposed Galileo program. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.
The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time-tagged signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data, for example, ephemeris parameters used to calculate satellite position and velocity, contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
The GPS receiver can distinguish signals from different satellites because the GPS system uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate navigation message data is encoded with a high-rate pseudo-random number (PRN) sequence that is unique for each satellite. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. GPS satellite L1 carrier signal is modulated by both the C/A and P codes. The C/A code is public and used by civilian GPS receivers, while the P code may be encrypted in which case it is only available to military equipment with a proper decryption key.
A GPS receiver typically has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the PRN code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal samples. During the acquisition stage, the code phase search step is, for example, a half-chip long for some navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from the receiver's local oscillator instability.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second (i.e. 1 bit/20 msec). This navigational data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames and frames. A sub-frame consists of 300 bits of data and is thus transmitted over 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A complete set of navigation data consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is known as the TLM word. The first eight bits of this TLM word, called the preamble, are always the same and used for frame synchronization (also called sub-frame synchronization). A Barker sequence is used as the preamble because of its excellent correlation properties. The second word of any frame is the HOW (Hand Over Word) and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity, with the last two bits of parity always being ‘0’s. These two ‘0’s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contain clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemerides are used to precisely determine the position and velocity of the GPS satellites. These ephemerides are normally uploaded every two hours and are valid typically for four hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and an almanac of satellites 25 to 32. The almanac is similar to the ephemeris but gives a less accurate orbit of the satellites and is typically valid for six months. The 3 to 10 words of the sub-frame 5 contain the almanac of satellites 1-24. The ephemeris of a particular signal from a satellite contains only the ephemeris of that satellite repeating in every frame. However, the almanac data of different satellites is broadcast in different frames (also called pages) of the navigation data signal of a given satellite. Thus the 25 frames transmit the almanac of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanac is included in the sub-frame 4. The almanac and ephemeris are used in the computation of the position and velocity of the satellites at a given time.
Accordingly, it is clear from the foregoing that the process of locking onto and synchronizing to signals from positioning system satellites, and particularly to being able to extracting meaningful data from such signals in, for example, an autonomous mode, is an important process before determining position and navigating using such signals can begin.
After acquiring enough satellites, a GPS receiver still needs a quite accurate (for example, at the several-millisecond accuracy level) TOW to calculate satellite positions and velocities, which are needed in a conventional navigation algorithm for a GPS receiver position fix. However, obtaining TOW from navigation message data bits of some live satellite signals typically requires at least bit synchronization, data demodulation, and frame synchronization processes. These processes take time and are even regarded as overly time-consuming in some applications.
Accordingly, a method and apparatus for quickly and effectively acquiring signals from positioning system satellites, obtaining TOW and delivering the first position fix remains desirable.
Augmentation of a Global Navigation Satellite System (GNSS) is a method of improving the navigation system's attributes, such as accuracy, reliability, and availability, by providing external information. There are many such systems in place and they are generally named or described based on how the GNSS sensor receives the external information. A Satellite Based Augmentation System (SBAS) is a system that supports wide-area or regional augmentation through the use of additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations used to collect GNSS signal data, located at accurately-surveyed points, master stations to calculate GNSS measurement error corrections, and a number of geostationary satellites for broadcasting the corrections. GNSS satellites broadcast differential correction data, and integrity data, and these signals themselves may also be utilized as an extra ranging signal for triangulation. Correction data may include: long term satellite position error; short term and long term satellite clock errors; and ionosphere correction data. WAAS, European EGNOS, and Japanese MSAS are all examples of a SBAS.
While SBAS designs and implementations may vary widely, with SBAS being a general term referring to any such satellite-based augmentation system, under the International Civil Aviation Organization (ICAO) rules a SBAS must transmit a specific message format and frequency which matches the design of the United States' Wide Area Augmentation System (WAAS). See RTCA, Inc., Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment, RTCA/DO-229 B, 1999.
SBAS is designed to improve GPS availability, accuracy and integrity. The present invention includes methods which extend the benefit of SBAS to provide faster GPS satellite signal acquisition and reduce the time to first fix (TTFF).